German Published Patent Application No. 40 12 109 relates to a device for monitoring the function of an electrical consumer, its output stage, its actuation, and the associated connections. Parallel to the output stage, an error detection logic circuit is connected which distinguishes between the following possible types of error: short circuit to ground, short circuit to the positive potential, and load drop, using a hardware-based comparison of the input and output potentials and using a reference potential.
German Published Patent Application No. 198 51 732 relates to a method and a device for monitoring at least one current control level for an electrical consumer. The current control level has at least one switching element and one current controller. A current which is specified by a pulse duty factor flows through the consumer, it being possible for the current controller to set the current to a setpoint value. To monitor the current control levels, the pulse duty factor and/or a variable derived from the pulse duty factor are compared to a threshold value, and an error is detected when there is a deviation from this value. The threshold value results from the particular corresponding variable which is present in at least one of the other current control levels. The ratio of the time in which the switching element is closed to the entire duration of actuation is referred to as the pulse duty factor. However, the ratio of the time in which the switching element is closed to the time in which the switching element is open, for example, may also be used.
Thus far, primarily two possibilities for monitoring the function of output stages having pulse width modulation (PWM) have been used in the related art: evaluating the status output from the intelligent output stage, or determining the status by feedback from the output stage output to a digital port. Neither possibility is suited for the active PWM operation. In the evaluation of the status signal from the intelligent output stage, the instantaneous status is first delivered on a delayed basis, the delay being specific to the output stage and possibly being greater than 1 ms, for example. During feedback to a digital input, the output signal must be scanned exactly at a specific point in time. This is achievable only with high computing power, up to the system-dependent limitations. When the two cited possibilities for monitoring the function of PWM output stages are used, it is possible to scan the output signal at the correct time only when the operating time from actuation of an output stage to the effect of the control signal at the output stage output is known.